Multigrid amplifier with constant ratio of cathode current to anode current



July 22, 1952 (3. WHITE El AL 2,604,552

ONSTANT R E. L. MULTIGR AMPLIFIER WITH C ATIO OF CA DE CURRENT TO ANODE CURRENT Filed April 50, 1947 117: A c. M117: 6 1701/9/70 1141-2 000/? ATTORN EY valve anode to earth.

Patented July 22, 1952 UNITED sTAr s PATENT oF cE 'MULTIGRJD AMPLIFIER WITH CONSTANT RATIO OF CATHODE CURRENT T ANODE CURRENT Eric Lawrence Casling White, Iver, and Roland William Elbourn, Southall, England, assignors to Electric & Musical Industries Limited, Hayes,

England, a company of Great Britain Application April 30, 1947, Serial No. 745,018 In Great Britain April 30,1946

This invention relates to thermionic valve circuits.

ance in the cathode-earth circuit. With such an arrangement it is necessary to ensure that the cathode current is the same as, or a constant fraction of, the anode current and when a triode valve is employed, which is always operated with.

the control electrode negativewith respect to the cathode so that no control electrode current flows, the cathode current is equal to the space current reaching the anode and except for very high frequencies where the shunting efiect of the cathodeearth stray capacity is appreciable compared with .the cathode feedback resistance, the potential measured at the cathode is a true measure of the anode current and is suitable for feedback purposes. However, it is often desirable in such a feedback circuit to use a tetrode or other multigrid valve and if, as is usually the case, one or tive so that current flows to this electrode, the

cathode potential is again no longer a true measure of the'anode current. Such effects are particularly noticeable when employing'valves in which the wires of the various electrodes. are aligned, in which case currentdrawn, for example, by a screening electrode isnot even approximately a constant fraction of the anode current. In circuits other than feedback circuits, similar difliculties may arise. One example of such a circuit is where impedances are provided in the anode and cathode circuits and it is desired to obtain a true pushpull output from the anode and cathode impedances.

The object of the present invention is to provide an improved circuit employing a thermionic valve having one or more electrodes between the cathode and anode which draw signal current but 5 Claims. (c1. 179-171) in which the variation of signal current in the cathode load is arranged to be the same as, or a substantially constant fraction of, the variation of the anode signal current.

According to'the invention a thermionic valve circuit is provided employing a valve having one or more electrodes between the cathode and anode which are likely to draw current, said valve also having an impedance in the anode circuit and an impedance'in the cathode circuit, and wherein in order that the cathode load current variation is maintained more nearly equal to, or more nearly a constant fraction of, the anode current variation, said one'or more electrodes which are likely to draw current are decoupled to the cathode. Where the invention is applied to a'valve in which the screening electrode'draws current, the feed resistance for said electrode is arranged to be large compared with the cathode impedance so that variations in the screen current are returned substantially entirely direct to the cathode and do not flow through the cathode load impedance. The impedance of the decoupling condenser employed at the lowest frequency to be considered in the signals passed bysaid valve should not be more than the cathode load impedance. If it is required for the valve to pass a D. C. signal component, the decoupling condenser may be replaced by a constant voltage device such as a neon discharge unit or floating battery or floating power unit. In the case where a valve is employed in which the control electrode is driven positive, said electrode may be driven from a cathode follower and the anode of the cathode follower is decouple'd'to the cathode of said valve. In order that the said invention may be clearly understood and readily carried into efiect, the same will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 illustrates a circuit arrangement according to one embodiment of the invention,

Figure 2 illustrates a circuit arrangement accoiling to another embodiment of the invention, an i Figure 3 illustrates a further embodiment of the invention. :7

The invention is shown in Figures 1 and 2 as being applied to a thermionic valve circuit employing a cathode impedance for obtaining negative feedback, it being required that the potential as measured at the cathode of said valve shall be a true measure of the anodecurrent of the valve. Referring now to Figure 1 of the drawings; the valve I is shown as a tetrode valve having a cathode 2, control electrode 3', screening .electrode 4 tial by connection to the positive terminal of the Since in source 8 through a feed resistance 9. operation the screening electrode 3' will draw current and in order that the potential variations measured at the cathode shall be a true measure of the anode current variations,- the screening electrode 4 is decoupled directly to the cathode 2 through a decoupling condenser In which is connected, as shown, between the screening electrode 4 and the upper end of the cathode resistance 6. The feed resistance 9 is arranged to be large compared with the cathode resistance 6 so that variations in the current drawn by the electrode 4 are returned substantially entirely direct to the cathode and do not flow through the resistance 5. The impedance of the decoupling condenser ID at the lowest frequency considered in the signals passedby the valve I should not be more than the resistance 6. In the example shown in Figure 1 the control electrode 3 may also be driven positive and this control electrode is also decoupled to the cathode 2. In order to effect such decoupling, signals applied to the valve I are fed thereto from input terminals II via a valve I2 which, in the example shown, is a triode valve, but which may be any other valve used as a triode, said valve [2 being provided with a cathode resistance I3 so that it functions as a cathode follower. The cathode of the valve I2 is supplied from a source of negative potential I 4. The'anode of the valve I2 is connected through a load resistance I5 to the positive terminal of the source of anode current 8 and the anode of the valve I2 is decoupled via a decoupling condenser I6 to the cathode 2 of the valve I. Providing the resistances I3 and I5 are large compared with the resistance 6, any current drawn by the control electrode 3 of the valve I is eifectively returned to the cathode 2 without passingthrough the resistance 6 via the cathodeanode path of the valve l2. The impedance of the decoupling condenser [6 at the lowest frequency considered in the signals applied to the terminals II should not be more than the resistance 6. It is of course assumed that the valve I2 is operated under such conditions that the control electrode of such valve does not pass current.

By virtue of the arrangement shown in Figure l, the potential measured at the cathode of the valve I is a true measure of the anode current of the valve I and the feedback potential produced across the resistance 6 is employed in well known manner to produce the required negative feedback. 7

Figure 2 of the drawings illustrates a further embodiment of the invention in which the decoupling condensers ID and I6 are replaced by neon discharge units and in which the feedback potential produced across the cathode resistance 6 is amplified and fed back to the valve I. In

Figure 2 signals to be amplified are applied to input terminals H and thence to a pair of valves I1 and I8, the cathodes of which are coupled together and connected to a cathode load re sistance I9, the anode of the valve I8 being connected via a load resistance 20 to the positive terminal of the source of anode current 8 and is connected, as shown, via a D. C. coupling circuit 4 comprising resistance 2!, condenser 22 and resistance 23 to the control electrode of the valve I2 which functions in the manner described with reference to Figure 1. ,Instead of theanode of the valve I2 being connected through a condenser IE to the cathode of the valve I, as in the arrangement shown in Figure 1, it is connected thereto via a neon discharge unit 24. Such form of" decoupling enables a D. C. signal component to be passed. Likewise, the screening electrode 4 of the valve I is also decoupled to the cathode 2 of the valve l via a neon discharge unit 25. In Figure 2 the feedback potential developed across the resistance I3 is applied to the control electrode of the valve I8 where it is amplified and ultimately applied to the valve I.

With the arrangements shown in Figures 1 and 2 of the drawings, if the control electrode of the valve I is driven positive, the gain of the cathode follower I8 suffers a slight but fairly sharp discontinuity at the point when current flows to the control electrode of the valve I and this is liable to cause a small but undesirable transient oscillation in the output particularly if the valve I2 is employed in a feedback loop as is the case with the circuit shown in Figure 2. In order to reduce such transient oscillation a circuit of the kind described in U. S. Patent 2,358,428 can be employed in place of the valve I2. Figure 3 of the drawings illustrates a circuit similar to that shown in Figure 1 but modified to include a pair of valves 26 and 21 in place of the valve I2. It will be seen that the twovalves 26 and 21 are arranged in a manner similar to that described in Figure l of the aforementioned patent. The valve 21 as well as the valve 26 is arranged to set up signals between the control electrode and cathode of the valve I under the control of signals applied to the control electrode of the valve 26. In the absence of signals the anode currents of the valves 26 and 21 are opposite in sense but when signals are applied to the control electrode of valve 26 the currents vary oppositely so that the signal contributions fed between the control electrode and cathode of valve I are in aiding sense. The effective slope of the circuit employing the two valves 26 and 21 is therefore much higher, compared with the case in which the single valve I2 is employed, so that the discontinuity in gain when current commences to flow to the control electrode 3 of the valve I is less marked. The valve 21 is provided, as shown, with a cathode resistance 28, the cathode of this valve 21 being decoupled to the cathode 2 of the valve I through decoupling condenser '29. This decoupling is in place of the decoupling of the anode of valve I2 in Figure 1 since, due to the employment of the two valves 26 and 21, the lower valve 21 will take greater current load than the upper valve 26. The anode of the valve 26 can also be decoupled by decoupling condenser 30 to the cathode 2 of the valve I to obtain better results.

Although the invention has been described above as applied to negative feedback circuits, it will be understood that the invention can be applied to other types of circuits.

' What we claim is:

1. In an electronic signal amplifying system for operation over a desired frequency range including a predetermined low frequency of signals, the combination'of a first electron discharge de vice having a cathode electrode, an anode electrode and control and screen electrodes, a cathode electrode feedback resistor for obtaining negative feedback, and a circuit arrangement for maintaining the cathode and anode currents of 5 said first electron discharge device at substantially a constant ratio comprising a first impedance means coupling said screen and cathode electrodes, a second electron discharge device having an anode circuit, a cathode circuit and a si nal input circuit, a direct connection between said cathode circuit and said control electrode, second impedance means coupling said anode circuit and said cathode electrode, direct-current energizing means having positive and negative terminals, a load impedance coupling said anode to said positive terminal, respective impedance elements having impedance values relatively large with respect to said feedback resistor coupling said screen electrode and said anode circuit to said positive terminal and coupling said cathode circuit to said negative terminal, said negative terminal and said feedback resistor being connected to a point of reference potential, and said first and second impedance means at said predetermined frequency of signals applied to said signal input circuit having impedance values not greater than the impedance of said feedback resistor.

2. In an electronic signal amplifying system, the combination 'as defined in claim 1, in which the first impedance means comprises a capacitor, and in which the sec-0nd impedance means comprises a capacitor.

3. In an electronic signal amplifying system, the combination as defined in claim 1, in which the first impedance means comprises a gas discharge device.

4. In an electronic signal amplifying system, the combination as defined in claim 3, in which the second impedance means comprises a gas discharge device.

5. In an electronic signal amplifying system, the combination as defined in claim 1, in which a third electron discharge device has an anode connected to the cathode circuit, a cathode capacitively coupled to the cathode electrode, a control element capacitively coupled to the anode circuit, and a resistive connection between said control element and said cathode.

ERIC LAWRENCE CASLING WHITE. ROLAND WILLIAM ELBOURN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,085,444 Perkins June 29, 1937 2,169,096 Greenberg Aug. 8, 1989 2,174,234 Cawein Sept. 26, 1939 2,198,464 Shepard Apr. 23, 1940 2,227,604 Smith Jan. 7, 1941 2,270,012 Shepard Jan. 13, 1942 FOREIGN PATENTS Number Country Date 508,697 Great Britain July 4, 1939 

